In the last video, we were talking about
this SunPower cell design, which uses an interdigitated back contact.
And we talked about all the nice things which
come along if you use this interdigitated back contact design.
First of all, your your solar cell, it looks very aesthetically appealing.
It looks all pitch black from the front - there is no metal fingers or
bus bars at the front. So it minimizes the shading.
And also some of the unique features that we
discussed, such as the use of n-type silicon,
these, localized p+ and n+ diffuision,
which are which are all which are all very native to this SunPower cell design.
Now in this video, what I want to do is discuss the process flow which
is needed to manufacture such a cell.
And, you'll realize that the devil is really in the details, when we,
so we talked about all the advantages that
you get by having these contacts at the back.
But the real problem or you know the real challenge comes along when you think
about how to manufacture these these these
cells, especially how to make these contacts at
the back, such that they're localized and they're interdigitated.
And that's where I think SunPower deserves a lot of credit for
commercializing this technology, and making the
price point come down at a point
at which it becomes competitive.
So, you know to give you an idea of
how this process flow can very quickly become complex,
I want to show you a sample,
an example of the process flow which can be
used to realize this realise this IBC kind of
a cell which SunPower makes.
So we start with, you know, similar to how
we start the processing for our traditional silicon solar cells.
to you. Make this make these textured texturing
on the top surface, very similar to how you do a, for a, for a conventional cell.
Over here you start with an n-type wafer instead of a p-type wafer.
Then you will do this you do this p-type diffusion
on the top to essentially form these surface field at the top.
So this is, you know,
also similar to how you do it for a conventional cell,
only you're, you're making a p-type diffusion instead of a.
doping it n+ type, at you would do in a conventional cell.
So the next process is essentially forming these localized diffusion.
So, what is you know, one of the simplest way you can
think of is to, you know, form a lithographic pattern, so do lithography
and open up these windows. And then essentially
you know, then essentially dope them with n-type dope
glass and then diffuse these n+ dopants
into your, into your device. So there is one lithotography step which
would be later required over here but
now you need to form
the p+ diffusion
as well and that needs
to be formed in these
spaces which are in, or which is
interdigitated between these n+ diffusion regions.
So you'll need to do another lithography step, so you'll need to
cover your n+ diffusion and open up this space
between uh, between these, the n+ diffusion. So you'll need a
second lithography step which is aligned with this,
which is aligned with this first lithography step.
And now you can form these p+ diffusion over here.
So you can form, these are my p+ diffusion, these are my n+ diffusion.
And then what I can do is I can essentially you know,
passivate the whole back surface with say silicon oxide or some other dielectric.
And then I can screen print these contacts,
so, I can screen print these
contacts and then I can fire them in such that
they connect to these local p+ and p+ diffusion.
But you know, you can see that I
required, I've used two lithography steps in this scheme.
And you know, anybody who works in a solar factory, they
hate this, they hate this L word, that it stands for lithography.
So every time you add a lithography step, it adds substantially
to the cost of your to the overall dollar per watt cost because
each of these lithography steps, you know you need to deposit a resist,
you need to clean the resist off and, you know, if you need to align the
first lithography step with the first, you need,
again you know, it becomes even more complex.
And it slows down your, your manufacturing line
substantially if you need, if you include any
lithography steps in your process flow.
So, you can see that your inclusion of these two lithography
steps makes this process flow very slow, and also makes it expensive.
Sometimes to, you know, to even improve, further improve their
cell characteristics, what is desired is that these back contacts,
Instead of placing them throughout the back surface, these are you
know, the whole back surface is and instead passivated with this silicon oxide.
And you open up these contact holes where you will make contact between your
metal and your semiconductors instead of making these contacts over the whole
line you make them now localized in these just these holes.
So this again requires a third lithography step.
So it
requires a third lithography steps, which again, adds to
your process flow cost as well as to its complexity.
So this is why it's, you know, it's
very expensive to make these interdigitated back contact cells.
What SunPower did and it should receive a lot of credit for that is
that it essentially simplified this process flow such that it minimizes
this amount of lithography steps.
So one of the, you know, one of the papers
that they published on this, back in the late, late 80s,
was to demonstrate this self-aligned process flow and that
reduce the number of lithography steps to to one.
And let me illustrate that, so how that is done.
So you start with the n-type wafer and
then you texture if
at the top,
just like you would do in a conventional process flow.
And then you form this p-type diffusion from the top.
So you form, that gives your surface
field which helps in minimizing this surface recombination.
And then you start working on making these localized,
localized diffusions at the back, so now this
is where it becomes really interesting, so the way this the SunPower paper described is
that you coat all of the back surface, you coat it with a layer that is essentially,
it constitutes of, of n-type dopants, so
you coat it with a PSG or a layer like that which contains these n-type dopants.
And now you do one lithography step, where you open up
these open up this layer which had n-type doping.
And then you also recess etch, so you etch your underlying silicon as well.
So you you etch this underlying silicon so
that it it forms these trenches inside your,
inside your entire, inside your silicon as well.
And now you coat this
so you know, we did, let's keep count of the lithography steps.
So we did one lithography step to essentially etch
this n-dopant layer and recess, recess into this silicon.
Now you coat the whole thing again, you coat the whole thing again
with another layer which has which is now a source of p-type dopant.
So you coat it
with your glass-like layer which is a source of p-type dopants.
So, all over here, you have this
this surface, especially that recessed surface over here.
It is now contacted with a source of p-type doping.
And now essentially, you fire this together, so you.
you essentially, you know, apply some thermal
budget such that you form these n+ diffusion
inside your silicon.
So you form this n+ diffusion in the region which is facing which is in
contact with this layer containing n diffu, n-type dopants.
And some of you found this inside this recess you found this p+ localized
diffusion because that surface is in contact
with this layer which contains p-type dopants.
And now you remove these layers, these layers which
were used to you know, source the dopant atoms.
So you remove these glass-like layers which
were which were essentially supplying these dopants.
So you are left with these junctions which have now been formed inside your silicon.
And you are left with this recess at the back.
And this recess is
is very important. as we'll see shortly.
So the next thing you do is now essentially you know,
the whole thing or.
you know, you essentially deposit aluminum on,
throughout this, throughout this back surface.
And when you deposit you know, aluminum usually when you deposit over a step
feature, so let's say you have a step feature step feature like this.
And you, ... let me use
the same color. So you have a step feature like this.
So this is assumed to be a view
of what's happening over here.
And now when you start depositing aluminium it would deposit something like
this, where layer by layer
layers of aluminium will deposit here, and this,
this, this surface will expand in this direction.
But that will prevent aluminum from depositing
on the edges of this bottom surface.
So if you look at the bottom surface
and this aluminium is depositing from this side.
So essentially it will automatically self-align these contacts
such that they don't short with each other.
So when you deposit
aluminum, it deposits in such a way such that
these metals, they hopefully don't short with each other.
And then you deposit, just to be double
sure, you deposit a layer of titanium over here.
So your deposit a layer of titanium
on the top. And then you etched back this
this aluminum. So this titanium layer with
it acts as a mask,
so even if there was some tita-, some aluminum
which got deposited over here, once you use this titanium layer
you can etch this back and you can make sure that this these metal contacts
which are connecting my n-type diffusion and my
p-type diffusions are not shorted with each other.
So now this would form my p bus bar, and this would form my n bus bar and
using this self-aligned process flow I can make sure
that you know, they're not shorted with each other.
Also I've used only one lithography step in this whole process flow.
And this is illustrated further, in this in this publication from SunPower,
and it shows how they have formed this interdigitated n+ and p+ contacts,
where this p+ contact is located, is
entrenched inside this trench which was created.
And this n+ contact, it's located at a
slightly higher feature as compared to this p+
contact, and you can see this this this recess etch going on over here as well.
So the key, over here, to separate these two layers
is this titanium layer which is deposited on the top, and using that and a
[INAUDIBLE]
mask you can etch into this aluminum.
So you can etch into this aluminum, and make sure that these p
bus bar, and these n bus bars are not shorted with each other.
So here I've illustrated a simpler process flow, where
you were able to achieve this using one lithography step.
More recently we know that Sunpower has eliminated all lithography steps.
So it uses
zero lithography steps. And it relies on screen printing of these
dopant layers.
So these instead of forming these doping by
diffusion, you essentially deposit or screen print these dopant
layers and then diffuse them in.
And more recently they also used this copper plating
at the back, at the back of these, to form these n+
and p+ interdigitated contacts, they use electroplating of copper,
which also helps in providing improved
mechanical strength, improved mechanical strength to this
solar cells.
So more details of this are not you know, this
detail of this process flow is not available in public literature.
But you get some hints of what's happening
by reading the patents which are being published.
But anyway, I hope that, you know, I have given you some
idea of how this process flow of forming these interdigitated back contact works.
And hopefully left with you with some food for
thought or some ideas where you can simplify it further.